1. Field of the Invention
The present invention relates generally to lithographic apparatus, an illumination system for use in lithographic apparatus contained therein, and to optical elements for manipulating a beam of radiation.
2. Description of the Prior Art
A lithographic apparatus is a machine that applies a desired pattern comprising features, structures, or line patterns onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of micro structure devices such as integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate contains a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Current structures on micro structure devices are often called ‘Manhattan structures’ since they are characterized by an orientation of the structures, line patterns, or features in mainly two perpendicular directions similar to a pattern of city streets. In current structure layout designs, these two directions are kept parallel to the respective boundary segments of a rectangular target exposure area on the substrate (die). By one convention, horizontal structures extend in an X-direction while vertical structures extend in a Y-direction. The width of the target portion is defined as the size of the rectangular area in the X-direction, and the height of the target portion is defined as the size of the rectangular area in the Y-direction. In lithographic scanners, the non-scanning direction is normally referred to as the X-direction while the scanning direction is referred to as the Y-direction.
A recent development in the layout design of micro structures is the use of features with an orientation other than strictly in the X- or Y-direction, i.e., line patterns extending in a direction that can make any angle between 0 and 90 degrees with respect to the X-direction. For example, the imaging of DRAM isolating structures may be optimized by using an angle between 20 and 30 degrees with respect to either the X- or Y-axis. Optimized imaging comprises for example an enhanced process latitude or increased depth of focus.
It is known that the imaging of structures with a certain orientation can be optimized by illuminating the mask with a projection beam containing rays of radiation with directions substantially perpendicular to the direction of the structures. The angular intensity distribution of the projection beam in a field plane such as the mask plane corresponds to a spatial intensity distribution in a pupil plane (the latter usually called “pupil shape” or just “pupil”). For example, the imaging of horizontal structures may be optimized by employing a projection beam with an associated pupil shape that has two high intensity regions separated from the optical axis in the Y-direction. The latter kind of pupil shape is referred to hereinafter as dipole. The corresponding illumination mode is referred to as dipole illumination, in this particular case as dipole Y illumination. Similarly, the imaging of structures that make an angle a with the X-axis can be optimized by dipole Y illumination in which the pupil is rotated over the same angle α. In general, when the imaging of certain structures is optimized by using a particular pupil shape, then if the structures are rotated around an axis also the pupil shape should be rotated around the same axis by an equal amount in order to maintain the same imaging performance. The pupil plane of the illumination system downstream of the integrator is also known in the art as the Fourier transform plane.
Current lithographic apparatus include an illumination system for providing a conditioned projection beam of radiation having desired dimensions, a desired spatial intensity distribution, and a desired angular intensity distribution at mask level. The illumination system includes an integrator to improve the uniformity of the projection beam with regard to spatial and angular intensity variations over the beam cross-section. The principle of an integrator is based on the creation of a plurality of secondary radiation sources or virtual secondary sources from a primary source, such that the beams originating from these secondary sources overlap at an intermediate field plane and average out. This averaging effect is called light integrating or light mixing.
One type of integrator is based on multiple reflections, referred to hereinafter as reflective integrator, and is embodied for example as a crystal rod made of quartz or calcium-fluoride (CaF2) or as a hollow waveguide, the faces of which are made of reflective material. This type of integrator generally has a rectangular (or square) cross-section and parallel side faces. Multiple secondary light sources are formed via multiple internal reflections (in the case of a rod type integrator) or via multiple specular reflections (in the case of a hollow waveguide type integrator) of the incoming radiation beam. Each reflective surface of the integrator ideally provides 100% reflection, but in practice the intensity of the reflected beam is decreased after each reflection due to residual surface defects or absorption.
An inherent property of a reflective integrator having a rectangular cross-section is that the angular intensity distribution of a beam exiting the integrator is forced to be symmetric with respect to the side faces of the integrator. i.e., the pupil shape of the beam that enters the integrator can be any shape, but due to the mixing of rays of radiation that has either made an even or an uneven number of reflections in the reflective integrator, the pupil shape of the beam that exits the integrator is mirror-symmetric with respect to two perpendicular axes parallel to the respective boundary segments of the rectangular cross-section of the reflective integrator, these axes normally oriented in the X- and Y-direction.
A problem with current lithographic apparatus including a reflective type of integrator (rod or hollow waveguide) arises when the imaging needs to be optimized for structures that extend in directions other than in the X- and/or Y-direction. For these other directions, a pupil shape of the projection beam which is non-mirror-symmetric with respect to the X- and Y-axes would be optimal. Such a pupil shape may also be referred to as a rotated mirror-symmetric pupil shape, or simply as a rotated pupil shape. Current illumination systems with reflective integrators are constructed and arranged such that they can provide mirror-symmetric pupil shapes such as for example annular, dipole-X, dipole-Y, quadrupole, hexapole, octopole. However, these systems cannot provide non-mirror-symmetric pupil shapes such as monopole, rotated dipole, tripole, rotated quadrupole.